Lithography was introduced to the electronics industry in the 1950's. The first integrated circuit was produced in 1960 by the firms Fairchild Semiconductor and Texas Instruments. Lithographic processes are used in both back plane and front plane display manufacturing. During the past 10 years, lithographic processes have been applied to new areas of precision patterning such as patterned synthesis of nucleic acid structures required for DNA testing.
In 1990 M. Rieger et al. described the value associated with direct write lithography, or maskless lithography. It has been broadly claimed that the elimination of masks will reduce process cost, increase process yield and enable the use of larger substrates. The use of laser patterning and light modulation to provide the ability to selectively pattern at high resolution is widely understood. U.S. Pat. No. 5,521,748 (Sarraf et al.) describes a light modulator for use with a laser or laser diode array such that the light from the laser is imaged on a light modulator having a row of light modulating elements, of either the reflectance or transmittance type. The light modulator is imaged onto light-sensitive materials and the image is scanned line by line on the light-sensitive materials. Control circuitry is provided to control the relative movement between the light modulator and the light-sensitive materials that may be either in planar form or wrapped around the circumference of a cylindrical drum.
Area based SLM have been described by Monk (The Digital Micro-mirror Device for Projection Display) in 1995. H. Kuck discloses the use area based spatial light modulators, lasers, optics and precision motion systems for the use in maskless lithographic systems in May of 1995. Kuck demonstrated the ability to produce sub micron features using this technique, but points out that to make this advantageous in the semiconductor industry one would need to increase the size of the SLMs and utilize precision air bearing motion systems. Hence the concept of utilizing lasers and spatial light modulators in direct write systems to eliminate the use of mask is clearly seen as advantageous.
There are, however, challenges that remain to be addressed to truly enable the process of maskless lithography on large substrates. Laser patterning will impart thermal energy to a substrate. The use of increasingly large linear or area array modulation only shifts the challenges of large mask production to the production of large lens arrays or large modulation assemblies. To minimize the manufacturing costs overall, one will need to utilize modulation with the appropriate number of channels. To meet the task time requirements, one may need to use parallel arrays, each at the appropriate number of channels. This application requires a shift in thinking away from the massively parallel model, which approximates large area masked processes. As this shift occurs, then one is faced with challenges involving time and space. First, time will elapse during the patterning process. Given that time, longer than that of a flash exposure, elapses, one must include in the challenges that of the dimensional stability of the substrate and the patterning mechanism, and in turn the dimensional relationship between the substrate and the patterning, light beam(s). The second challenge involves the spatial relationship of the multiple spatial light modulators or printhead assemblies with one another as well as the spatial relationship between these assemblies and the substrate.
Current fabrication processes operate on Gen VIII sized glass substrates. The industry agrees that this is either at or close to the limit for substrate size. Moving beyond this size prompts discussion of patterning on flexible support and possibly manufacturing in web based processes.
U.S. Pat. No. 6,251,550 (Ishikawa) describes a maskless lithography system, which utilizes an area exposure system. He mentions both LCD displays used as electronic masks and micro-mirror devices. It is advantaged in that the traditional alignment step involving a lateral shift of a mask can occur through electronic means. The traditional shifting of a mask in X and or Y may be seen as a global alignment process. As the display industry moves to fabrication on larger substrates and flexible support there is a need for a means of dynamically detecting and compensating for local changes in the registration of the patterning channels with patterns on the substrate in process.